G4RandomDirection.hh

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// 
// ------------------------------------------------------------
//      GEANT 4 class header file 
// ------------------------------------------------------------
// Class description:
//
// Function returning a unit 3-vector homogeneously randomised over 4pi
// solid angle. It can be used in any particle scattering methods
// instead of:
//   z=R1, x=SQRT(1-R1*R1)*SIN(2*pi*R2), y=SQRT(1-R1*R1)*COS(2*pi*R2)
// providing more performant results.

// History:
//    19.10.17 E. Tcherniaev, use of G.Marsaglia (1972) method
//    06.04.17 E. Tcherniaev, added G4RandomDirection(cosTheta)
//    15.03.16 E. Tcherniaev, removed unnecessary if and unit()
//    18.03.08 V. Grichine, unit radius sphere surface based algorithm
//      ~ 2007 M. Kossov, algorithm based on 8 Quadrants technique
//
// ------------------------------------------------------------
#ifndef G4RANDOMDIR_HH
#define G4RANDOMDIR_HH

#include <CLHEP/Units/PhysicalConstants.h>

#include "globals.hh"
#include "Randomize.hh"
#include "G4ThreeVector.hh"

// G.Marsaglia (1972) method
inline G4ThreeVector G4RandomDirection()
{
  G4double u, v, b;
  do {
    u = 2.*G4UniformRand() - 1.;
    v = 2.*G4UniformRand() - 1.;
    b = u*u + v*v;
  } while (b > 1.);
  G4double a = 2.*std::sqrt(1. - b);
  return G4ThreeVector(a*u, a*v, 2.*b - 1.);
}

inline G4ThreeVector G4RandomDirection(G4double cosTheta)
{
  G4double z   = (1. - cosTheta)*G4UniformRand() + cosTheta;
  G4double rho = std::sqrt((1.+z)*(1.-z));
  G4double phi = CLHEP::twopi*G4UniformRand();
  return G4ThreeVector(rho*std::cos(phi), rho*std::sin(phi), z);
}

#endif  /* G4RANDOMDIR_HH */